It is well-known that magnetic tape recorders may be used, for example, in television recording systems for the recording of video information and related audio signals. Normally, in this type of system, two distinct recording tracks were utilized, one for the video signal and one for the audio. Similarly, it is common practice in the art of radar data recording, to record video and trigger pulses on a first magnetic tape track and to record antenna coordinate (bearing) information on a second tape track, the coordinate data being modulated with audio message data, if desired.
More specifically, in the field of radar data recording, it has been proposed previously to combine the radar video and trigger pulses to form a composite signal which is recorded on a first tape track of a multiple track recorder, with antenna position data signals (in either scalar or synchro format) being separately recorded on a second track of the recorder. An example of this type of radar data recorder is disclosed in the U.S. patent to Buchholz et al, U.S. Pat. No. 3,634,887, issued Jan. 11, 1972. In U.S. Pat. No. 3,725,912, issued Apr. 3, 1973, Buchholz et al subsequent disclose a radar data recording/reproducing system wherein a composite signal is generated for recording on a single tape track, including not only the summed video and trigger pulses but also digital representation of radar antenna elevation angle. In this later patent, the video information is blanked during the interval that the digital code of elevation angle is being recorded. Trigger identification is accomplished by converting the radar triggers to pulses of controlled polarities, prior to summing them with the video.
In order to overcome the deficiency of these and other prior art techniques for the recording and playback of radar and similar types of informational signals/data, the present invention provides an improved method and apparatus to facilitate the recording, on a single magnetic tape track for example, of all such radar information and in a manner which preserves system timing and assures faithful reproduction of the radar display upon playback. Additionally, the proposed system of the present invention obviates the need to rely upon precise timing of the trigger pulse recording, in order to identify the various triggers encountered in radar system operation, and it also enables the radar coordinate information to be recorded on the same recording track as the video information without loss of video, thus improving significantly the quality of the reproduced radar display. As will be described in more detail hereinafter, the proposed radar recording/reproducing apparatus of the present invention is also compatible with fixed or variable PRF type radars, without requiring time consuming manual set-up or expensive delay lines.
In one embodiment of the present invention, the recorded composite signal contains radar video and trigger markers and is chopped by a bi-phase, multi-bit data signal comprising (1) a four-bit synchronizing sequence which identifies the beginning of a data word, (2) a two-bit trigger identification code (which necessarily precedes the associated trigger marker in the composite signal), and (3) 20-bits of radar bearing or other related data. The bi-phase code has two fundamental frequencies, the bit rate frequency and twice the bit rate frequency. Thus, a logical zero has, for example, a zero crossing or transition at the beginning of each bit cell, whereas a logical one has a transition both at the beginning and at the center of each bit cell. As a result, the data may be decoded by merely detecting the zero crossing transitions. As will be described in detail later, each time a start of sweep (SOS) or end of sweep (EOS) trigger is received, a data word is formatted in the bi-phase code, consisting of the word synchronization sequence, trigger identification code and bearing data components, etc., as noted above.
In a second embodiment, the radar video and trigger markers frequency modulate a first carrier signal, which is later combined with a second carrier which has previously been frequency modulated with the bi-phase data, including word synchronization, trigger identification, radar coordinates, etc., to form the composite signal for recording. Here again, following frequency demodulation, recovery of the trigger identification code occurs prior to recovery of the trigger marker, with the data being decoded by detection of the zero crossing transitions of the data bits.
Without in any way attempting to limit the spirit or scope of the present invention, it will be clear to those skilled in the art that the proposed monotrack recording/playback system of the present invention has utility in diverse recording/reproducing applications such as, for example, both single and multiple track radar recording/playback, radar information transmission applications, and, in general, to the recording and playback of any wideband information which necessarily must be synchronized to specific events; i.e., to the trigger pulses in radar applications. By way of examples, a radar data recording/reproducing system such as is proposed in accordance with the present invention could be utilized for analyzing radar information obtained over a long period of time; for use in the training of radar operators; and, to provide a permanent and retrieval record of radar information relative to vehicle navigation, for example, which can be replayed as desired, for study or investigative purposes in the event of an accident, etc. It should also be understood at this time that the specific bi-phase code, bit widths, number of bits used, etc. can obviously be altered depending upon the requirements of practice, without in any manner departing from the spirit or scope of the present invention.
In view of the foregoing discussion, one object of the present invention is to provide a system wherein all signals necessary to record and playback signals, such as those required to reproduce a radar display can be accommodated on a single wideband magnetic tape track.
Another object of the present invention is to provide a monotrack radar data recording/playback system compatible with fixed or variable PRF radars.
A further object of the present invention is to provide a monotrack radar data recording/playback system employing recorded, positively identified radar triggers and recorded bearing coordinates which are synchronized with the recorded video signals.
A further object of the present invention is to provide apparatus capable of recording on a single tape track the video, triggers and coordinate information from diverse types of radar systems and capable of being implemented on commercially available color TV video recording apparatus.